1. Field of the Invention
The present invention relates to an active noise control system which produces a signal that is interfere with and attenuates an uncomfortable noise generated in the passenger compartment of a vehicle by the operation of the engine or under the running condition thereof, the signal being equal in amplitude and opposite in phase with the noise. More particularly, the present invention is directed to an active noise control system which prevents an abnormal acoustic noise from being generated due to an improper noise reduction operation resulting from an abnormal output signal from a microphone for sensing a residual noise level.
2. Description of the Related Art
Conventionally known in the prior art is a method for sensing the abnormal level of an active noise control system using an output signal from the active noise control system and a signal obtained in accordance with the behavior of a speaker for radiating the output signal into the air (e.g., see Japanese Laid-Open Patent Publication No. Hei 6-250671). FIG. 6 is a view illustrating the configuration of a conventional active noise control system disclosed in Japanese Laid-Open Patent Publication No. Hei 6-250671.
The active noise control system shown in FIG. 6 operates to cancel a noise released through a muffler from an engine or a noise source. A controller portion 9 produces a noise-canceling signal, which is in turn converted from digital to analog at a D-A (Digital to Analog) converter 4 and then filtered through a low-pass filter 3 to remove unwanted high frequency harmonic components therefrom, finally supplied to a power amplifier 2. The noise-canceling signal that has been power amplified at the power amplifier 2 is radiated through a speaker 1 into the air as an acoustic canceling-signal, which is then interfere with and cancels the noise from the muffler. The cancellation may result in a residual noise, which is then converted by a microphone 5 into an electric signal to be supplied to an amplifier 6 as an error signal. The error signal that has been amplified at the amplifier 6 is filtered through a low-pass filter 7 to remove unwanted high frequency harmonic components therefrom, and then supplied to an A-D (Analog to Digital) converter 8. The A-D converter 8 converts the supplied analog signal into a positive or negative digital signal with respect to an initial voltage setting (e.g., the DC bias voltage for the low-pass filter 7) employed as a reference value (0). The error signal “e” that has been quantized and converted from analog to digital at the A-D converter 8 is supplied to the controller portion 9 to produce a noise-canceling signal. The controller portion 9 incorporates a DSP (Digital Signal Processor) or a discrete micro-processing unit for processing digital signals. The DSP is provided with an adaptive filter for performing main processing, in which the noise-canceling signal is adaptively produced in accordance with a noise demonstrative signal (reference signal) resulting from the pulsation frequency of the engine and the error signal, thereby making it possible to reduce a stationary low-frequency noise generated by the noise source.
The active noise control system is provided with an abnormal level detection portion 13 for sensing its own abnormal level. The abnormal level detection portion 13 is supplied with abnormal level detection signals delivered from each portion of the active noise control system. When processing these abnormal level detection signals to find an abnormal level, the abnormal level detection portion 13 produces a signal for resetting the controller portion 9, a signal for reducing the level of the acoustic canceling-signal, and a signal for turning off a power supply switch 14 of the controller portion 9 itself, thereby stopping the function of producing the noise-canceling signal.
Now, the abnormal level detection signal for the abnormal level detection portion 13 to sense the abnormal level of the active noise control system itself will be described in more detail below. The abnormal level detection signal shown by (1) in FIG. 6 serves to sense the abnormal level based on a strong vibration of the diaphragm of the speaker 1. A large vibrational amplitude of the diaphragm causes a switch, which is provided on the reverse side of the diaphragm of the speaker 1, to be turned on or off to produce a signal, which is then compared with the reference signal, thereby sensing the abnormal level. That is, the abnormal level can be sensed because the large vibrational amplitude of the diaphragm of the speaker 1 means that the active noise control system is delivering an excessive output level.
The abnormal level detection signal shown by (2) in FIG. 6 serves to sense the abnormal level in accordance with an abnormal increase in temperature of the voice coil of the speaker 1. The speaker 1 is provided with a thermocouple near the voice coil to produce a signal resulting from a thermo-electromotive force being converted into a voltage, and the signal is compared with a reference voltage, thereby sensing the abnormal level. That is, the abnormal level can be sensed because an abnormal increase in temperature of the voice coil means that an excessive output signal current is flowing.
The abnormal level detection signal shown by (3) in FIG. 6 serves to sense the abnormal level in accordance with a change in magnetic flux density caused by an output current from the power amplifier 2 to the speaker 1. A magnetic flux density detector is provided on a cable through which the output current flows to the speaker 1, and the output signal from the magnetic flux density detector is rectified and smoothed to produce a signal, which is in turn compared with the reference voltage to thereby sense the abnormal level. That is, the abnormal level can be sensed because detecting a change in magnetic flux density means that an abnormal low-cycle current of a high output level is flowing through the speaker 1.
The abnormal level detection signal shown by (4) in FIG. 6 serves to sense the abnormal level in accordance with the level of the noise-canceling signal to be supplied to the power amplifier 2. The output signal from the low-pass filter 3 to be supplied to the power amplifier 2 is branched to produce a rectified and smoothed signal, which is in turn compared with the reference voltage to thereby sense the abnormal level. That is, the abnormal level can be sensed because the noise-canceling signal level indicative of an abnormal value means that the expected maximum value is exceeded.
The abnormal level detection signal shown by (5) in FIG. 6 serves to sense the abnormal level in accordance with the level of a signal produced by removing the noise-canceling signal from the signal to be supplied to the power amplifier 2. The output signal from the low-pass filter 3 to be supplied to the power amplifier 2 is branched and then allowed to pass through a band-stop filter for removing the frequency band of the noise-canceling signal, thereby providing a band-stop signal. The band-stop signal is rectified and smoothed to produce a signal, which is in turn compared with the reference voltage to thereby sense the abnormal level. That is, the abnormal level can be sensed because the band-stop signal level indicative of an abnormal value means that frequency components other than those of the noise-canceling signal are contained.
The abnormal level detection signal shown by (6) in FIG. 6 serves to sense the abnormal level through the phase comparison between a signal to be supplied to the power amplifier 2 and the output signal from the low-pass filter 7. The abnormal level is sensed in accordance with the level of an output signal from a phase comparator which compares the phase of a signal branched from the output signal from the low-pass filter 3 to be supplied to the power amplifier 2 and the phase of the output signal from the low-pass filter 7. That is, the abnormal level can be sensed because the level of the output signal from the phase comparator indicative of an abnormal value means that the signals no longer hold the relationship of being equal in frequency and opposite in phase.
However, the conventional active noise control system allows the controller portion 9 to stop the function of producing the noise-canceling signal as a result of the speaker 1 or the power amplifier 2 having already operated, or after the abnormal level has been determined in accordance with the value of the noise-canceling signal that has been already delivered as a signal. The system allows the abnormal acoustic noise to continually radiate into the air for the period of time immediately after the abnormal level has actually occurred until the abnormal level detection portion 13 determines the abnormal level. Accordingly, the conventional system may cause the user to possibly hear the abnormal acoustic noise during that period of time. Particularly, when the error signal “e” from the microphone 5 to be supplied to the controller portion 9 is indicative of the abnormal level, the controller portion 9 adaptively computes an abnormal level, providing an improper noise reduction effect. Additionally, in the worst case, it is highly possible that the computed result of the adaptive filter does not converge but diverges. In this case, until the abnormal level detection portion 13 determines the abnormal level, an output signal having an approximately maximum level that the controller portion 9 can possibly provide is delivered successively. Thus, the conventional system may cause significant discomfort to the user.